The cable leaving a diesel generator is one of the more common places a rating is got wrong, because the kVA on the nameplate is not the number that sizes the cable. What sizes it is the full-load current the set delivers, and that depends on the voltage as well as the kVA.
Start with the full-load current
For a three-phase set the full-load current is the kVA multiplied by 1000, divided by 1.732 times the line voltage. At 415 V that is about 1.39 A for every kVA. The cable and the outgoing breaker are then chosen above this figure, never exactly on it.
| DG rating (kVA) | Indicative full-load current at 415 V, 3-phase |
|---|---|
| 25 | about 35 A |
| 62.5 | about 87 A |
| 125 | about 174 A |
| 250 | about 348 A |
| 500 | about 696 A |
| 1000 | about 1390 A |
These are the current figures only. The cable size that carries them depends on how the cable is installed and how hot its surroundings are, so the current is the starting point, not the answer.
Then apply the installation realities
A DG cable usually sits in a hot environment, inside an acoustic canopy or a plant room, so the ambient derating is heavier than for a cable in open air. Where the current is large, the run is often split into two or more cables in parallel per phase, and each of those is derated again for grouping. The common construction is a 3.5-core armoured cable, aluminium in the larger sizes and copper where space or terminations call for it, with the armour earthed at both ends.
Check the run length last
A set feeding a panel some distance away needs a voltage-drop check on top of the current rating. Size on the current-carrying figure with its derating, then confirm the drop over the actual length is inside the limit, and take the larger conductor of the two. APAR supplies LV XLPE power cables in copper and aluminium up to 1000 sq mm for generator feeders, and high-ampacity 105°C XLPE cables where a hot location eats into the rating.